1. Field of the Invention
The present invention relates to a liquid ejection head which ejects a liquid, and particularly relates to a layer which protects a heat generating portion in a liquid ejection head which ejects a liquid utilizing thermal energy.
2. Description of the Related Art
An ink ejection method described in U.S. Pat. No. 4,723,129, and in U.S. Pat. No. 4,740,796, is a method which ejects ink by utilizing thermal energy to cause an air bubble to form in the ink, and therefore enables a high speed, high image quality printing. Also, as this method is appropriate for colorization and downsizing, in recent years it has become a mainstream ink jet printing method.
A general configuration of a liquid ejection head using this method is one which includes a plurality of ejection openings, liquid paths communicating with the ejection openings, and electro-thermal converting elements which generate thermal energy utilized for ejecting ink. The electro-thermal converting element is configured to include a heating resistor and electrodes for supplying power thereto. Further, the electro-thermal converting elements are coated with a protective layer which has an electrical insulation property, so that an insulation property is secured for each electro-thermal converting element. Each liquid path communicates with a common liquid chamber, and ink is supplied to the common liquid chamber from an ink tank. The ink supplied to the common liquid chamber is led into each liquid path, and then forms a meniscus in the vicinity of the ejection opening to be held. The electro-thermal converting elements are selectively driven in this condition, and thermal energy is generated by the driven electro-thermal converting element. The generated thermal energy applies heat to ink rapidly through an ink contact portion (heat application portion) located above the electro-thermal converting element, to generate a bubble. Then, ink can be ejected by pressure of generated bubble.
The heat application portion of this type of liquid ejection head (hereafter called simply the “head”), as well as being exposed to a high temperature by the above described thermal energy generation, is multiply subjected to a physical action, such as the shock of a cavitation accompanying an expansion and a contraction of a bubble in ink, and a chemical action caused by the ink. Normally, a protective layer is provided in the heat application portion in order to protect the electro-thermal converting element from these effects. Conventionally, a protective layer of a tantalum film having a thickness of 0.2 to 0.5 μm is provided, in which the film is comparatively resistant to the shock of the cavitation and to the chemical action due to the ink.
Also, with the heat application portion, a phenomenon occurs whereby a coloring material, an additive, and the like contained in the ink are broken down to the molecular level by being heated to a high temperature to be changed to a hardly-soluble matter, and physically adsorbed onto the protective layer. This phenomenon is called cogation.
When a hardly-soluble organic matter or inorganic matter is adsorbed onto the protective layer due to the cogation, the transfer of heat from the heating resistor to the ink becomes uneven and the bubble generation becomes unstable. Therefore, the tantalum film, on which it is comparatively difficult for the cogation to occur, is generally used as the protective layer.
Hereafter, a description will be given, referring to FIG. 1, for conditions of generation and disappearance of a bubble in ink at the heat application portion.
A curved line (a) shown in FIG. 1 indicates a temporal change of surface temperature of a protective layer from a time point at which a drive voltage is applied to the heating resistor, in the case that the drive voltage Vop is taken to be 1.3×Vth (here, Vth indicates a bubble generation threshold voltage of the ink), a drive frequency 6 kHz, and a pulse width 5 μsec. On the other hand, a curved line (b) indicates by volume a development state of the generated bubble, likewise from the point at which the drive voltage is applied to the heating resistor. As shown by the curved line (a), a rise in temperature starts from the voltage being applied, reaches a peak temperature a little later than a set, predetermined pulse width (time) (because it takes a little longer for the heat from the heating resistor to reach the upper portion of the protective layer). After then, the temperature decreases due mainly to heat diffusion. Meanwhile, as shown by the curved line (b), the development of a bubble starts from a time point at which the protective layer surface temperature is around 300° C. and, after attaining a maximum volume, decreases its volume and disappears. These changes are caused repeatedly in an actually used head. In this way, it can be understood that the protective layer surface rises to a temperature of around, for example, 600° C. along with the generation of the bubble, and that the ink jet printing accompanies a high temperature thermal action. Then, this type of high temperature thermal action gives rise to the problem of cogation occurring.
In response to this problem, there are conventionally known countermeasures which make it difficult for cogation to occur by using ink containing a dye with a high heat resistance, or by using ink in which the amount of impurities in the dye is reduced by carrying out a sufficient refining. However, there are problems in that the manufacturing cost of the ink increases accordingly, the types of dye which can be used are limited, and the like.
That the above described problem arising due to the cogation are solved by a method differing from that which suppresses the occurrence of cogation is described in Japanese Patent Laid-Open No. 2008-105364. That is, in the document, it is described that, as well as using iridium (Ir) or ruthenium (Ru) for the protective layer which comes into contact with the ink, the protective layer is caused to be eluted so that the cogation is removed, by carrying out an electrolytic reaction.
However, although the method described in Japanese Patent Laid-Open No. 2008-105364 can effectively remove the cogation, regarding the above described protective layer formed on the head substrate, there is a difficulty relating to adhesion of the protective layer with a resin layer of a path wall or the like which are formed on the protective layer. As a result, a problem develops in that a detachment may occur between the members.
In particular, in the case of using an elongated (in particular, 0.5 inches or more) liquid ejection head in order to contribute to the recent speeding-up of printing, a comparatively large distortion occurs due to a difference in linear expansion coefficient between head composing members, stress of a resin layer forming the path wall and ejection opening, and the like. In this case, if there is a difficulty with the adhesion between the protective layer and the resin layer, the detachment may occur between the members. Also, in the case of using ink containing an additive for increasing the light resistance or gas resistance of the ink ejected onto the printing medium, this type of ink has an adverse effect on the interface between the members, and there may be the detachment occurring between the resin layer for forming the path wall or the like, and the protective layer. Furthermore, also in the case of providing an organic layer, for improving the adhesion, on the protective layer, it may happen that a detachment will occur around the interface between the adhesion improvement layer and protective layer. As a result, for example, there is a possibility that ink will seep onto the substrate, causing corrosion of the wiring, and it will be difficult to secure long-term quality and reliability of the liquid ejection head.